After Wang and Zhang initially investigated the dechlorination effect of the nanosized zerovalent iron (nZVI) in treatment of organochlorine compounds in 1997 [Wang, C. B. and Zhang, W. X. “Synthesizing nanoscale iron particles for rapid and complete dechlorination of TCE and PCBs.” Environmental Science & Technology 31.7 (1997): 154-2156.], the nanosized zerovalent iron, due to its large specific surface area and high reaction activity, has become a research focus in such fields as in situ groundwater remediation and treatment of sewage chloroorganics; in order to effectively reduce agglomeration of nanoparticles and control high head pressure, resin, membrane and activated carbon are adopted as the support for the nanomaterial; namely, the supported nZVI is more widely used in practical application. However, despite the general high activity of the supported nZVI, it presents low efficiency and incompleteness in degrading recalcitrant chloroorganics such as chlorobenzene and polychlorinated biphenyls (PCBs); besides, some products emerged during the degradation process present even higher toxicity (for example, when nZVI is utilized to reduce some organic halogens, the products may bear higher toxicity than the original pollutants and become even more difficult to be further degraded by iron). In order to further improve the reaction activity of the supported nZVI and to extend its service life, researchers introduce a second metal, for example, palladium (Pd), nickel (Ni), copper (Cu) and Platinum (Pt), onto the support; the loaded second metal on the surface of nZVI can engender satisfactory catalytic effect and high efficiency by accelerating the fuel cell reaction (with numerous microfuel cells being formed in the system) that facilitates nZVI losing electrons and lowers down the activation energy. Currently, the conventional method for preparing the supported bimetallic nanocatalyst goes as follows: first, adopting a reductive to reduce the high-valent iron into nanosized zerovalent iron and loading it on the support, then making use of the reducibility of the zerovalent iron and loading the ionic second metal on the support through reduction reaction. The second metal in the bimetallic nanocatalyst prepared with this method is loaded on the surface of the zerovalent iron through deposition process. As the catalytic reduction progresses, the iron on the support corrodes away, and the second metal loaded on the surface of the iron is also washed away, which consequently greatly lowers down the catalytic activity of the composite catalyst [Zhu, B. W. and Lim, T. T. 2007. “Catalytic Reduction of Chlorobenzenes with Pd/Fe Nanoparticles: Reactive Sites, Catalyst Stability, Particle Aging, and Regeneration.” Environ. Sci. Technol 41 (2007): 7523-7529.].
In 2009, (China) Nanjing University successfully applied for two patents: “Nanocomposite Resin Loaded with Zerovalent Iron for Catalyst Degradation of Pollutants and the Preparation Method Thereof” (Application Number: 200910028413.X; Publication Number: CN101474560) and “Iron-loaded Bimetallic Nanocomposite Cation Exchange Resin and the Preparation Method Thereof” (Application Number: 200910028414; Publication Number: CN101497051A). The bimetallic materials mentioned in this two patents are prepared as follows: firstly, loading the iron precursor on the support and reducing the precursor with NaBH4 or KBH4; the supported nanosized zerovalent iron is therefore obtained; then, soaking the iron-loaded resin in the saline solution of a second metal so that the reducibility of zerovalent iron can be utilized to load the ionic second metal through reduction reaction. The two metals prepared in this way are interdependently distributed within the support in that the second metal deposits on the surface of the zerovalent iron and the two metals constitute a core-shell structure.
Currently, the conventional method for preparing the supported bimetallic nanocatalyst goes as follows: adopting firstly a reductive to reduce the high-valent iron into nanosized zerovalent iron and loading it on the support, then making use of the reducibility of the zerovalent iron and loading the ionic second metal on the support. The second metal of the bimetallic nanocatalyst prepared with this method is directly loaded on the surface of the zerovalent iron. As the catalytic reduction progresses, the iron on the support corrodes away, and the second metal loaded on the surface of the iron is also washed away, which consequently greatly lowers down the catalytic activity of the composite catalyst.
Up to present, there is not any documentation reporting a method that utilizes the ion exchange effect to load a second metal on the support and to realize independent distribution of the zerovalent iron and the second metal so that the catalytic stability of the material can be improved.